The present invention is directed to a process of flash fusing xerographic images. More specifically, the present invention is directed to processes for flash fusing xerographic images with energy pulses having a duration exceeding one millisecond without resulting in image de-enhancement. Image de-enhancement occurs when in one embodiment the process is performed by means of energy pulses of two to five milliseconds in conjunction with a toner composition containing a colloidal silica powder.
Flash fusing is one of several methods available for permanently affixing toner images to substrates in the xerographic process. The process consists of the application of a rapid pulse of energy to the unfixed image, which causes the toner particles to melt and fuse to the substrate. Generally, flash fusing systems are designed to produce short flash pulses of energy of from about 0.5 to about 1 millisecond. Shorter pulses tend to produce excess surface temperature on the toner, which causes the toner to vaporize. Longer pulses result in image de-enhancement, the magnitude of which is dependent upon the fluidity of the toner. The de-enhancement observed when longer pulses are applied occurs because the flash energy is delivered to the toner so slowly that the toner material at the toner-paper interface remains in a very viscous state. This condition precludes wetting and spreading of the toner on the paper, but allows for coalescence of the toner particles. The net effect of applying a longer pulse is to allow a toner pile to remain in the coalescence stage of fusing at energies which, if applied in shorter pulses, would have driven the toner to a much higher temperature and resulted in little or no de-enchancement.
All flash fusing systems using energy pulses of one millisecond or less have one major disadvantage, the power supplies require capacitors, which add considerable cost, weight, and size to the system. A flash fusing system with circuitry designed such that power is drawn directly from a 117 volt alternating current line would eliminate the need for capacitors and would thus enable flash fusing in small copiers, that is those with, for example, speeds of 12 copies per minute. Such a system is disclosed in copending application U.S. Ser. No. 872,328, filed June 9, 1986, entitled "Electrophotographic Reproduction Machine With Document Exposure System Directly Coupled to AC Line Input", the disclosure of which is totally incorporated herein by reference. One characteristic of a flash fusing system drawing its power directly from a 117 VAC line is that it is capable of delivering flash pulses of a duration no shorter than approximately 3 to 5 milliseconds.
U.S. Pat. No. 4,698,290 the disclosure of which is totally incorporated herein by reference, teaches a process for reducing the energy required for flash fusing of electrostatographic images. More specifically, the process of the copending application, which reduces the energy required for flash fusing by providing a waxy, low viscosity layer at the toner-substrate interface during fusing, entails developing an image with a toner composition comprising resin particles, pigment particles, and wax, transferring the image to a substrate, and flash fusing the transferred image. An example of a toner composition disclosed in this application comprises 70 percent by weight of a polyester resulting from the condensation reaction of dimethylterephthalate, 1,3-butenediol, and pentaerythritol, 10 percent by weight of carbon black, and 20 percent by weight of polypropylene, to which was added 0.5 percent by weight of Aerosil R972.RTM.. This copending application does not, however, illustrate flash fusing processes wherein the fusing times are between 2 and 5 milliseconds.
Copending application U.S. Ser. No. 809,359, filed Dec. 16, 1985, entitled "Flash Fusing Process With Prespheroidized Toner", the disclosure of which is totally incorporated herein by reference, also discloses a process for reducing the energy required for flash fusing of electrostatographic images. The process of this application comprises the prespheroidization of the toner particles by heat spheroidization, which reduces image de-enhancement and reduces the required flash fusing energy. The application does not, however, disclose a flash fusing process wherein the fusing time is between 2 and 5 milliseconds. A specific toner composition illustrated in this copending application comprises 90 percent by weight of a polyester resulting from the reaction of 2,2-bis(4-hydroxyisopropoxy phenol) propane and fumaric acid, and 10 percent by weight of carbon black, to which was added 0.5 percent by weight of Aerosil R972.RTM.. According to the disclosure of this application, the aerosil functions primarily as a charging source.
Another reference, U.S. Pat. No. 3,900,588, discloses the use of silica compositions as toner additives to eliminate toner film buildup on the carrier particles, and to reduce the impaction of toner particles on the carrier. The silica compositions also function to maintain the stability of the developer's triboelectric properties, according to the teachings of this patent.
Also, toner compositions containing silica compositions are well known in the xerographic art. For example, U.S. Pat. No. 2,986,521 discloses a developer powder for use in electrostatic printing comprising particles of a low melting organic solid coated with colloidal silica. According to the teaching of this patent, the developer composition reduces image de-enhancement by improving the flow characteristics of the unfused developer powder.
In U.S. Pat. No. 4,288,517, a toner composition for electrostatic photography, which comprises a toner powder containing a base resin, a coloring agent, and a silica powder such as aerosil is illustrated. The surfaces of the toner particles are coated with the base resin of the toner powder. According to the teachings of this patent, the toner composition prevents toner-carrier deterioration from mechanical wear during multiple imaging and prevents formation of toner films on the carrier and the photoconductor. The toner composition also provides stable powder and electric characteristics, and improves the process of toner-carrier mixing.
Additionally, U.S. Pat. No. 4,301,228 discloses a developer which includes carrier particles, electrically insulative toner particles, and electrically insulative fine particles composed of a metallic oxide, such as silica. The silica particles adhere to the surfaces of both the toner particles and the carrier particles preventing toner and carrier from adhering to each other after long periods of use.
Also, U.S. Pat. No. 4,533,616 illustrates a developer composition containing a toner and a microencapsulated additive capable of gradual release into the developer during the development process. The additive may be a microcapsule particle with a core of colloidal silica and a wall of a high polymer surrounding the core. This developer composition has high stability, high durability, good charging characteristics, and improved flow characteristics of the dry powder.
Furthermore, U.S. Pat. No. 4,555,467 discloses a one-component developer comprising toner particles and flow-improving granules composed of a coloidal silica. The developer has high durability and fixability, stores well, does not adhere to photoreceptor surfaces, and has good flow characteristics as a dry powder.
Japanese Patent Publication No. 51-81623 discloses a negatively charged toner consisting of small amounts of silica dispersed in a resin. The toner is characterized by a negative polarity when in contact with a carrier, and a lack of adhesion between individual particles. The toner provides images of uniform density and does not adhere to the developing apparatus. A similar toner is disclosed in Japanese Patent Publication No. 60-107036, said toner containing a binding resin and a fine silica powder which is mixed with the resin, and has good fluidity even though it contains a small amount of silica powder.
Although the above documents disclose toner compositions suitable for their intended purposes, they do not illustrate a process for the flash fusing of images that avoids image de-enhancement when energy pulses greater than one millisecond are used. In the above references, the addition of a silica composition improves the flow characteristics of the dry toner powder or functions as a charging source; none of these references, however, refer to the problem of image de-enhancement during flash fusing processes using energy pulses of 2 to 5 milliseconds.